Split-ladder band n77 filter using transversely-excited film bulk acoustic resonators

ABSTRACT

Band N77 bandpass filters include a first plurality of transversely-excited film bulk acoustic resonators (XBARs) on a first chip comprising a first rotated YX-cut lithium niobate piezoelectric plate having a thickness less than or equal to 535 nm, and a second plurality of XBARs on a second chip comprising a second rotated YX-cut lithium niobate piezoelectric plate having a thickness greater than or equal to 556 nm. A circuit card is coupled to the first chip and the second chip. The circuit card includes conductors for making electrical connections between the first chip and the second chip.

RELATED APPLICATION INFORMATION

This patent claims priority from provisional patent application62/983,403, titled MULTIPLE PIEZOELECTRIC THICKNESS XBAR FILTERS, filedFeb. 28, 2020.

NOTICE OF COPYRIGHTS AND TRADE DRESS

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. This patent document may showand/or describe matter which is or may become trade dress of the owner.The copyright and trade dress owner has no objection to the facsimilereproduction by anyone of the patent disclosure as it appears in thePatent and Trademark Office patent files or records, but otherwisereserves all copyright and trade dress rights whatsoever.

BACKGROUND Field

This disclosure relates to radio frequency filters using acoustic waveresonators, and specifically to filters for use in communicationsequipment.

Description of the Related Art

A radio frequency (RF) filter is a two-port device configured to passsome frequencies and to stop other frequencies, where “pass” meanstransmit with relatively low signal loss and “stop” means block orsubstantially attenuate. The range of frequencies passed by a filter isreferred to as the “pass-band” of the filter. The range of frequenciesstopped by such a filter is referred to as the “stop-band” of thefilter. A typical RF filter has at least one pass-band and at least onestop-band. Specific requirements on a pass-band or stop-band depend onthe specific application. For example, a “pass-band” may be defined as afrequency range where the insertion loss of a filter is better than adefined value such as 1 dB, 2 dB, or 3 dB. A “stop-band” may be definedas a frequency range where the rejection of a filter is greater than adefined value such as 20 dB, 30 dB, 40 dB, or greater depending onapplication.

RF filters are used in communications systems where information istransmitted over wireless links. For example, RF filters may be found inthe RF front-ends of cellular base stations, mobile telephone andcomputing devices, satellite transceivers and ground stations, IoT(Internet of Things) devices, laptop computers and tablets, fixed pointradio links, and other communications systems. RF filters are also usedin radar and electronic and information warfare systems.

RF filters typically require many design trade-offs to achieve, for eachspecific application, the best compromise between performance parameterssuch as insertion loss, rejection, isolation, power handling, linearity,size and cost. Specific design and manufacturing methods andenhancements can benefit simultaneously one or several of theserequirements.

Performance enhancements to the RF filters in a wireless system can havebroad impact to system performance. Improvements in RF filters can beleveraged to provide system performance improvements such as larger cellsize, longer battery life, higher data rates, greater network capacity,lower cost, enhanced security, higher reliability, etc. Theseimprovements can be realized at many levels of the wireless system bothseparately and in combination, for example at the RF module, RFtransceiver, mobile or fixed sub-system, or network levels.

High performance RF filters for present communication systems commonlyincorporate acoustic wave resonators including surface acoustic wave(SAW) resonators, bulk acoustic wave (BAW) resonators, film bulkacoustic wave resonators (FBAR), and other types of acoustic resonators.However, these existing technologies are not well-suited for use at thehigher frequencies and bandwidths proposed for future communicationsnetworks.

The desire for wider communication channel bandwidths will inevitablylead to the use of higher frequency communications bands. Radio accesstechnology for mobile telephone networks has been standardized by the3GPP (3^(rd) Generation Partnership Project). Radio access technologyfor 5^(th) generation mobile networks is defined in the 5G NR (newradio) standard. The 5G NR standard defines several new communicationsbands. Two of these new communications bands are n77, which uses thefrequency range from 3300 MHz to 4200 MHz, and n79, which uses thefrequency range from 4400 MHz to 5000 MHz. Both band n77 and band n79use time-division duplexing (TDD), such that a communications deviceoperating in band n77 and/or band n79 uses the same frequencies for bothuplink and downlink transmissions. Bandpass filters for bands n77and n79must be capable of handling the transmit power of the communicationsdevice. WiFi bands at 5 GHz and 6 GHz also require high frequency andwide bandwidth. The 5G NR standard also defines millimeter wavecommunication bands with frequencies between 24.25 GHz and 40 GHz.

The Transversely-Excited Film Bulk Acoustic Resonator (XBAR) is anacoustic resonator structure for use in microwave filters. The XBAR isdescribed in U.S. Pat. No. 10,491,291, titled TRANSVERSELY EXCITED FILMBULK ACOUSTIC RESONATOR. An XBAR resonator comprises an interdigitaltransducer (IDT) at least partially disposed on a thin floating layer,or diaphragm which is or includes a layer of a single-crystalpiezoelectric material. The IDT includes a first set of parallelfingers, extending from a first busbar and a second set of parallelfingers extending from a second busbar. The first and second sets ofparallel fingers are interleaved. A microwave signal applied to the IDTexcites a shear primary acoustic wave in the piezoelectric diaphragm.XBAR resonators provide very high electromechanical coupling and highfrequency capability. XBAR resonators may be used in a variety of RFfilters including band-reject filters, band-pass filters, duplexers, andmultiplexers. XBARs are well suited for use in filters forcommunications bands with frequencies above 3 GHz.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an exemplary split-ladder band-passfilter.

FIG. 2 has a schematic plan view and two schematic cross-sectional viewsof a transversely-excited film bulk acoustic resonator (XBAR).

FIG. 3 is an expanded schematic cross-sectional view of a portion of theXBAR of FIG. 2.

FIG. 4 is a graphic illustrating a shear primary acoustic mode in anXBAR.

FIG. 5 is a graph of the resonance frequency of an XBAR as a function ofpiezoelectric plate thickness for four different dielectric thicknesses.

FIG. 6 is a graph of the resonance frequency of an XBAR as a function ofequivalent diaphragm thickness for four different dielectricthicknesses.

FIG. 7 is a graph of the anti-resonance frequency of an XBAR as afunction of piezoelectric plate thickness for four different dielectricthicknesses.

FIG. 8 is a graph of the anti-resonance frequency of an XBAR as afunction of equivalent diaphragm thickness for four different dielectricthicknesses.

FIG. 9 is a schematic diagram of another exemplary split-ladder band N77band-pass filter.

FIG. 10 is a table of resonator parameters for the exemplary band N77band-pass filter of FIG. 8.

FIG. 11 is a graph of the magnitude of the input/output transferfunction S21 for an exemplary spit-ladder band N77 band-pass filter.

FIG. 12 is a flow chart of a process for fabricating a chipincorporating XBARs.

FIG. 13 is a flow chart of a process for fabricating a split-ladderfilter.

Throughout this description, elements appearing in figures are assignedthree-digit or four-digit reference designators, where the two leastsignificant digits are specific to the element and the one or two mostsignificant digit is the figure number where the element is firstintroduced. An element that is not described in conjunction with afigure may be presumed to have the same characteristics and function asa previously-described element having the same reference designator.

DETAILED DESCRIPTION Description of Apparatus

FIG. 1 is a schematic circuit diagram of an exemplary band-pass filter100 using five XBARs X1-X5. The filter 100 may be, for example, a bandN77 band-pass filter for use in a communication device. The filter 100has a conventional ladder filter architecture including three seriesresonators X1, X3, X5 and two shunt resonators X2, X4. The three seriesresonators X1, X3, X5 are connected in series between a first port P1and a second port P2. The filter 100 is bidirectional and either portmay serve as the input or output of the filter. The two shunt resonatorsX2, X4 are connected from nodes between the series resonators to ground.All the shunt resonators and series resonators are XBARs.

Each of the resonators X1 to X5 has a resonance frequency and ananti-resonance frequency. In over-simplified terms, each resonator iseffectively a short circuit at its resonance frequency and effectivelyan open circuit at its anti-resonance frequency. Each resonator X1 to X5creates a “transmission zero”, where the transmission between the in andout ports is very low. Note that the transmission at a “transmissionzero” is not actually zero due to energy leakage through parasiticcomponents and other effects. The three series resonators X1, X3, X5create transmission zeros at their respective anti-resonance frequencies(where each resonator is effectively an open circuit). The two shuntresonators X2, X4 create transmission zeros at their respectiveresonance frequencies (where each resonator is effectively a shortcircuit). In a typical band-pass filter using acoustic resonators,resonance frequencies of the shunt resonators are positioned below thepassband of the filter and the anti-resonance frequencies of the shuntresonators are within the passband. Resonance frequencies of the seriesresonators are within the passband and the anti-resonance frequencies ofthe series resonators are positioned above the passband.

The filter 100 uses a “split-ladder” architecture in which the seriesresonators X1, X3, X5 are fabricated on a first chip 110 and the shuntresonators X2, X4 are fabricated on a second chip 120. The first chip110 and the second chip 120 are mounted on a circuit board 130 whichprovides interconnections between the first and second chips 110, 120.As will be described subsequently, the use of the split-ladderarchitecture allows the use of a different thickness of piezoelectricmaterial for the series and shunt resonators.

Referring now to FIG. 2, the structure of XBARs will be described infurther detail. FIG. 2 shows a simplified schematic top view andorthogonal cross-sectional views of a an XBAR 200. The XBAR 200 is madeup of a thin film conductor pattern formed on a surface of apiezoelectric plate 210 having a front surface 212 and a back surface214. The front and back surfaces are essentially parallel. “Essentiallyparallel” means parallel to the extent possible within normalmanufacturing tolerances. The piezoelectric plate is a thinsingle-crystal layer of a piezoelectric material such as lithiumniobate, lithium tantalate, lanthanum gallium silicate, gallium nitride,or aluminum nitride. The piezoelectric plate is cut such that theorientation of the X, Y, and Z crystalline axes with respect to thefront and back surfaces is known and consistent. In the examplespresented in this patent, the piezoelectric plates are rotated YX-cut.However, XBARs may be fabricated on piezoelectric plates with othercrystallographic orientations including rotated Z-cut and rotated Z-cut.

The back surface 214 of the piezoelectric plate 210 is attached to asurface 222 of the substrate 220 except for a portion of thepiezoelectric plate 210 that forms a diaphragm 215 spanning a cavity 240formed in the substrate 220. The cavity 240 has a perimeter defined bythe intersection of the cavity and the surface 222 of the substrate 220.The portion of the piezoelectric plate that spans the cavity is referredto herein as the “diaphragm” due to its physical resemblance to thediaphragm of a microphone. As shown in FIG. 2, the diaphragm 215 iscontiguous with the rest of the piezoelectric plate 210 around all ofthe perimeter 245 of the cavity 240. In this context, “contiguous” means“continuously connected without any intervening item”.

The substrate 220 provides mechanical support to the piezoelectric plate210. The substrate 220 may be, for example, silicon, sapphire, quartz,or some other material or combination of materials. The back surface 214of the piezoelectric plate 210 may be attached to the substrate 220using a wafer bonding process. Alternatively, the piezoelectric plate210 may be grown on the substrate 220 or otherwise attached to thesubstrate. The piezoelectric plate 210 may be attached directly to thesubstrate or may be attached to the substrate 220 via one or moreintermediate material layers.

The cavity 240 is an empty space within a solid body of the resonator200. The cavity 240 may be a hole completely through the substrate 220(as shown in Section A-A and Section B-B) or a recess in the substrate220 (not shown). The cavity 240 may be formed, for example, by selectiveetching of the substrate 220 before or after the piezoelectric plate 210and the substrate 220 are attached.

The conductor pattern of the XBAR 200 includes an interdigitaltransducer (IDT) 230. An IDT is an electrode structure for convertingbetween electrical and acoustic energy in piezoelectric devices. The IDT230 includes a first plurality of parallel elongated conductors,commonly called “fingers”, such as finger 236, extending from a firstbusbar 232. The IDT 230 includes a second plurality of fingers extendingfrom a second busbar 234. The first and second pluralities of parallelfingers are interleaved. The interleaved fingers overlap for a distanceAP, commonly referred to as the “aperture” of the IDT. Thecenter-to-center distance L between the outermost fingers of the IDT 230is the “length” of the IDT.

The term “busbar” refers to the conductors that interconnect the firstand second sets of fingers in an IDT. As shown in FIG. 2, each busbar232, 234 is an elongated rectangular conductor with a long axisorthogonal to the interleaved fingers and having a length approximatelyequal to the length L of the IDT. The busbars of an IDT need not berectangular or orthogonal to the interleaved fingers and may havelengths longer than the length of the IDT.

The first and second busbars 232, 234 serve as the terminals of the XBAR200. A radio frequency or microwave signal applied between the twobusbars 232, 234 of the IDT 230 excites a primary acoustic mode withinthe piezoelectric plate 210. As will be discussed in further detail, theprimary acoustic mode is a bulk shear mode where acoustic energypropagates along a direction substantially orthogonal to the surface ofthe piezoelectric plate 210, which is also normal, or transverse, to thedirection of the electric field created by the IDT fingers. Thus, theXBAR is considered a transversely-excited film bulk wave resonator.

The IDT 230 is positioned on the piezoelectric plate 210 such that atleast a substantial portion of the fingers of the IDT 230 are disposedon the diaphragm 215 of the piezoelectric plate that spans, or issuspended over, the cavity 240. As shown in FIG. 2, the cavity 240 has arectangular shape with an extent greater than the aperture AP and lengthL of the IDT 230. A cavity of an XBAR may have a different shape, suchas a regular or irregular polygon. The cavity of an XBAR may have moreor fewer than four sides, which may be straight or curved.

For ease of presentation in FIG. 2, the geometric pitch and width of theIDT fingers is greatly exaggerated with respect to the length (dimensionL) and aperture (dimension AP) of the XBAR. An XBAR for a 5G device willhave more than ten parallel fingers in the IDT 210. An XBAR may havehundreds, possibly thousands, of parallel fingers in the IDT 210.Similarly, the thickness of the fingers in the cross-sectional views isgreatly exaggerated in the drawings.

FIG. 3 shows a detailed schematic cross-sectional view of the XBAR 200.The piezoelectric plate 210 is a single-crystal layer of piezoelectricmaterial having a thickness ts. ts may be, for example, 100 nm to 1500nm. When used in filters for 5G NR and WiFI bands from 3.3 GHZ to 7 GHz,the thickness ts may be, for example, 250 nm to 700 nm.

A front-side dielectric layer 314 may be formed on the front side of thepiezoelectric plate 210. The “front side” of the XBAR is the surfacefacing away from the substrate. The front-side dielectric layer 314 hasa thickness tfd. The front-side dielectric layer 314 may be formed onlybetween the IDT fingers (see IDT finger 338 a). The front sidedielectric layer 314 may also be deposited over the IDT fingers (see IDTfinger 338 b). A back-side dielectric layer 316 may be formed on theback side of the piezoelectric plate 210. The back-side dielectric layer316 has a thickness tbd. The front-side and back-side dielectric layers314, 316 may be a non-piezoelectric dielectric material, such as silicondioxide or silicon nitride. tfd and tbd may be, for example, 0 to 500nm. tfd and tbd are each typically less than one-half of the thicknessis of the piezoelectric plate. tfd and tbd are not necessarily equal,and the front-side and back-side dielectric layers 314, 316 are notnecessarily the same material. Either or both of the front-side andback-side dielectric layers 314, 316 may be formed of multiple layers oftwo or more materials.

The IDT fingers 338 a, 338 b may be one or more layers of aluminum, asubstantially aluminum alloys, copper, a substantially copper alloys,beryllium, gold, molybdenum, or some other conductive material. Thin(relative to the total thickness of the conductors) layers of othermetals, such as chromium or titanium, may be formed under and/or overthe fingers to improve adhesion between the fingers and thepiezoelectric plate 210 and/or to passivate or encapsulate the fingers.The busbars (232, 234 in FIG. 2) of the IDT may be made of the same ordifferent materials as the fingers. As shown in FIG. 3, the IDT fingers338 a, 338 b have rectangular or trapezoidal cross-sections. The IDTfingers may have some other cross-sectional shape.

Dimension p is the center-to-center spacing or “pitch” of the IDTfingers, which may be referred to as the pitch of the IDT and/or thepitch of the XBAR. Dimension w is the width or “mark” of the IDTfingers. The IDT of an XBAR differs substantially from the IDTs used insurface acoustic wave (SAW) resonators. In a SAW resonator, the pitch ofthe IDT is one-half of the acoustic wavelength at the resonancefrequency. Additionally, the mark-to-pitch ratio of a SAW resonator IDTis typically close to 0.5 (i.e., the mark or finger width is aboutone-fourth of the acoustic wavelength at resonance). In an XBAR, thepitch p of the IDT is typically 2 to 20 times the width w of thefingers. In addition, the pitch p of the IDT is typically 2 to 20 timesthe thickness tp of the piezoelectric plate 210. The width of the IDTfingers in an XBAR is not constrained to one-fourth of the acousticwavelength at resonance. For example, the width of XBAR IDT fingers maybe 500 nm or greater, such that the IDT can be fabricated using opticallithography. The thickness tm of the IDT fingers may be from 100 nm toabout equal to the width w. The thickness of the busbars (232, 234 inFIG. 2) of the IDT may be the same as, or greater than, the thickness tmof the IDT fingers.

FIG. 4 is a graphical illustration of the primary acoustic mode ofinterest in an XBAR. FIG. 4 shows a small portion of an XBAR 400including a piezoelectric plate 410 and three interleaved IDT fingers430. A radio frequency (RF) voltage is applied to the interleavedfingers 430. This voltage creates a time-varying electric field betweenthe fingers. The direction of the electric field is primarily lateral,or parallel to the surface of the piezoelectric plate 410, as indicatedby the arrows labeled “electric field”. Since the dielectric constant ofthe piezoelectric plate is significantly higher than the surroundingair, the electric field is highly concentrated in the plate relative tothe air. The lateral electric field introduces shear deformation, andthus strongly excites a shear-mode acoustic mode, in the piezoelectricplate 410. Shear deformation is deformation in which parallel planes ina material remain parallel and maintain a constant distance whiletranslating relative to each other. A “shear acoustic mode” is anacoustic vibration mode in a medium that results in shear deformation ofthe medium. The shear deformations in the XBAR 400 are represented bythe curves 460, with the adjacent small arrows providing a schematicindication of the direction and magnitude of atomic motion. The degreeof atomic motion, as well as the thickness of the piezoelectric plate410, have been greatly exaggerated for ease of visualization. While theatomic motions are predominantly lateral (i.e. horizontal as shown inFIG. 4), the direction of acoustic energy flow of the excited primaryshear acoustic mode is substantially orthogonal to the surface of thepiezoelectric plate, as indicated by the arrow 465.

An acoustic resonator based on shear acoustic wave resonances canachieve better performance than current state-of-the artfilm-bulk-acoustic-resonators (FBAR) and solidly-mounted-resonatorbulk-acoustic-wave (SMR BAW) devices where the electric field is appliedin the thickness direction. In such devices, the acoustic mode iscompressive with atomic motions and the direction of acoustic energyflow in the thickness direction. In addition, the piezoelectric couplingfor shear wave XBAR resonances can be high (>20%) compared to otheracoustic resonators. High piezoelectric coupling enables the design andimplementation of microwave and millimeter-wave filters with appreciablebandwidth.

The resonance frequency of an XBAR is determined by the thickness of thediaphragm and the pitch and mark (width) of the IDT fingers. Thethickness of the diaphragm, which includes the thickness of thepiezoelectric plate and the thicknesses of front-side and/or back-sidedielectric layers, is the dominant factor determining the resonancefrequency. The tuning range provided by varying the pitch and/or mark islimited to only a few percent. For broad bandwidth filters such as bandN77 and band N79 band-pass filters, the tuning range provided by varyingthe pitch is insufficient to provide the necessary separation betweenthe resonance frequencies of the shunt and the anti-resonancefrequencies of the series resonators.

U.S. Pat. No. 10,491,192 describes the use of a front-side dielectricfrequency setting layer formed only over shunt resonators to extend thebandwidth capability of XBAR filters. The dielectric frequency settinglayer increases the thickness of the diaphragms of the shunt resonatorsand thus reduces the resonance frequency of the shunt resonatorsrelative to the series resonators. However, there is a practical limitto the thickness of a frequency setting layer because a thick dielectriclayer fosters additional spurious modes that may degrade filterperformance.

FIG. 5 is graph 500 of the relationship between the resonance frequency,piezoelectric plate thickness, and top-side dielectric thickness forrepresentative XBARs using lithium niobate piezoelectric plates withEuler angles of [0°, β, 0°], where 0°<β≤60°, as described in U.S. Pat.No. 10,790,802. For historical reasons, such plates are commonlyreferred to as “rotated YX-cut” where the “rotation angle” is β+90°.Band N77 filters may preferably use 120° to 128° degree rotated YX-cutpiezoelectric plates, which is to say plates with Euler angles [0°, 30°to 38°, 0°].

The solid line 510 is a plot of the dependence of resonance frequency onpiezoelectric plate thickness without a front-side dielectric layer. Theshort-dash line 520 is a plot of the dependence of resonance frequencyon piezoelectric plate thickness with a front-side dielectric layerthickness of 30 nm. The dot-dash line 530 is a plot of the dependence ofresonance frequency on piezoelectric plate thickness with a front-sidedielectric layer thickness of 60 nm. The long-dash line 540 is a plot ofthe dependence of resonance frequency on piezoelectric plate thicknesswith a front-side dielectric layer thickness of 90 nm. In all cases, thepiezoelectric plate is 128-degree YX-cut lithium niobate, the IDT pitchis 10 times the piezoelectric plate thickness, the mark/pitch ratio is0.25, and the IDT electrodes are aluminum with a thickness of 1.25 timesthe piezoelectric plate thickness. The dielectric layers are SiO₂. Thereis no back-side dielectric layer.

The dash-dot-dot line 550 marks the lower edge of band N77 at 3300 MHz.From FIG. 5, one can see that a resonance frequency of 3300 MHz can beobtained by different combinations of piezoelectric plate thickness anddielectric thickness. For example, with no dielectric layer, apiezoelectric plate thickness of 561 nm results in a resonance frequencyof 3300 MHz. Alternatively, a piezoelectric plate thickness of 516 nmwith a 90 nm dielectric layer also provides a resonance frequency of3300 MHz.

As previously described, in a ladder filter circuit, shunt resonatorsprovide transmission zeros at frequencies below the lower edge of thefilter passband. To this end, the resonance frequencies of the shuntresonators of a band N77 filter must be less than 3300 MHz. Exactly howmuch less than 3300 MHz depends on the filter specifications, theQ-factors of the shunt resonators, and allowances for manufacturingtolerances and temperature variations. The filled circle 560 representsXBARs using 580 nm piezoelectric plate thickness with a 20 nm front-sidedielectric layer, as used in an exemplary filter to be describedsubsequently.

From FIG. 5 it can also be seen that 90 mm layer of SiO₂ has the sameeffect on resonance frequency as roughly a 45 nm (561 nm-516 nm) changein the thickness of the lithium niobate piezoelectric plate. Thethicknesses of the lithium niobate piezoelectric plate and thefront-side SiO₂ layer can be combined to provide an equivalent thicknessof the XBAR diaphragm as follows:

teqr=tp+kr(tfsd)  (1)

where teqr is the “LN-equivalent” thickness (i.e. the thickness oflithium niobate having the same resonance frequency) of the diaphragmfor shunt resonators. tp and tfsd are the thickness of the piezoelectricplate and front-side dielectric layer as previously defined, and kr isproportionally constant for shunt resonators. kr depends on the materialof the front-side dielectric layer. When the front-side dielectric layeris SiO₂, kr is approximately 0.59.

FIG. 6 is a graph 600 of the dependence of resonance frequency onLN-equivalent diaphragm thickness. The data shown in FIG. 6 is the sameas the data shown in FIG. 5, with LN-equivalent thickness calculatedusing formula (1) with k=0.59. The composite line 610 is made up of theresonance frequency data points for four different dielectric (oxide)thicknesses. These data points form a reasonably continuous curve.

The data in FIG. 6 is specific to XBARs with the IDT pitch equal to 10times the piezoelectric plate thickness, a preferred range of the pitchof an XBAR is 6 to 12.5 times the IDT pitch (see U.S. Pat. No.10,637,438). Electromechanical coupling decreases sharply for pitch lessthan 6 time the piezoelectric plate thickness, reducing the differencebetween the resonance and anti-resonance frequencies. Increasing thepitch above 12.5 times the piezoelectric plate thickness reducescapacitance per unit resonator area with little benefit in terms ofelectromechanical coupling or frequency change.

Reducing the pitch to 6 times the piezoelectric plate thicknessincreases resonance frequency by about 4.5% compared to an XBAR withpitch equal to 10 times the piezoelectric plate thickness. Increasingthe pitch to 12.5 times the piezoelectric plate thickness reducesresonance frequency by about 1.1% compared to an XBAR with pitch equalto 10 times the piezoelectric plate thickness. Conversely, an XBAR withpitch equal to 6 times the piezoelectric plate thickness will need a4.5% thicker diaphragm to have the same resonance frequency as an XB ARwith pitch equal to 10 times the piezoelectric plate thickness. An XBARwith pitch equal to 12.5 times the piezoelectric plate thickness willneed a 1.1% thinner diaphragm to have the same resonance frequency as anXBAR with pitch equal to 10 times the piezoelectric plate thickness.

The dash-dot-dot horizontal line 650 marks the lower edge of band N77 at3300 MHz. The dash-dot vertical line 655 marks an LN-equivalentthickness of 563 nm, which is the thickness that provides a resonancefrequency of 3300 MHz. The LN-equivalent plate thickness would bereduced to about 556 nm if the pitch is increased to 12.5 time thepiezoelectric plate thickness. In a band N77 filter using a ladderfilter circuit, the diaphragms of all of the shunt resonators will haveLN-equivalent thicknesses of greater than 556 nm. Typically, at leastone shunt resonator will have a LN-equivalent diaphragm thickness of 565nm to 600 nm to provide a transmission zero a at resonance frequency 50MHz to 200 MHz less than the lower band edge (i.e. at 3100 MHZ to 3250MHz).

The LN-equivalent thickness of the diaphragm of the shunt resonatorsneed not be the same. One or more additional dielectric layer may beformed over a subset of the shunt resonators to lower their resonancefrequencies (for example to increase attenuation in a stop band belowthe lower band edge). While there is no absolute upper limit on thethickness of a front-side dielectric layer, XBARs with tfsd/tp>0.30 tendto have substantial spurious modes. Thus a practical upper limit onLN-equivalent diaphragm thickness is about (1+0.3×0.59)(556 nm)=655 nm.

FIG. 7 is a graph 700 of the relationship between the anti-resonancefrequency, piezoelectric plate thickness, and top-side dielectricthickness for representative XBARs. The solid line 710 is a plot of thedependence of anti-resonance frequency on piezoelectric plate thicknesswithout a front-side dielectric layer. The short-dash line 720 is a plotof the dependence of anti-resonance frequency on piezoelectric platethickness with a front-side dielectric layer thickness of 30 nm. Thedot-dash line 730 is a plot of the dependence of anti-resonancefrequency on piezoelectric plate thickness with a front-side dielectriclayer thickness of 60 nm. The long-dash line 740 is a plot of thedependence of anti-resonance frequency on piezoelectric plate thicknesswith a front-side dielectric layer thickness of 90 nm. In all cases, thepiezoelectric plate is 128-degree YX-cut lithium niobate, the IDT pitchis 10 times the piezoelectric plate thickness, the mark/pitch ratio is0.25, and the IDT electrodes are aluminum with a thickness of 1.25 timesthe piezoelectric plate thickness. The dielectric layers are SiO₂. Thereis no back-side dielectric layer.

The dash-dot-dot line 750 marks the upper edge of band N77 at 4200 MHz.From FIG. 7, one can see that an anti-resonance frequency of 4200 MHzcan be obtained by different combinations of piezoelectric platethickness and dielectric thickness. For example, with no dielectriclayer, a piezoelectric plate thickness of 518 nm results in ananti-resonance frequency of 4200 MHz. Alternatively, a piezoelectricplate thickness of 480 nm with a 90 nm dielectric layer also provides ananti-resonance frequency of 4200 MHz.

As previously described, in a ladder filter circuit, series resonatorsprovide transmission zeros at frequencies above the upper edge of thefilter passband. To this end, the anti-resonance frequencies of theseries resonators of a band N77 filter must be greater than 4200 MHz.Exactly how much greater than 4200 MHz depends on the filterspecifications, the Q-factors of the series resonators, and allowancesfor manufacturing tolerances and temperature variations includingtemperature increases due to power consumed in the filter duringtransmission. The filled circles 760, 765 represent XBARs using 455 nmpiezoelectric plate thickness with a 20 nm and 60 nm, respectively,front-side dielectric layers, as used in the exemplary filter to bedescribed subsequently.

From FIG. 7 it can also be seen that a 90 mm layer of SiO₂ has the sameeffect on resonance frequency as roughly a 38 nm (518 nm−480 nm) changein the thickness of the lithium niobate piezoelectric plate. Thethicknesses of the lithium niobate piezoelectric plate and thefront-side SiO₂ layer can be combined to provide an equivalent thicknessof the XBAR diaphragm as follows:

teqa=tp+ka(tfsd)  (1)

where teqa is the “LN-equivalent” thickness (i.e. the thickness oflithium niobate having the same resonance frequency) of the diaphragmfor series resonators. tp and tfsd are the thickness of thepiezoelectric plate and front-side dielectric layer as previouslydefined, and ka is proportionally constant for series resonators. kadepends on the material of the front-side dielectric layer. When thefront-side dielectric layer is SiO₂, ka is approximately 0.45.

FIG. 8 is a graph 800 of the dependence of anti-resonance frequency onLN-equivalent diaphragm thickness. The data shown in FIG. 8 is the sameas the data shown in FIG. 7, with LN-equivalent thickness calculatedusing formula (1) with ka=0.45. The composite line 810 is made up of theanti-resonance frequency data points for four different dielectricthicknesses. These data points form a reasonably continuous curve, withthe exception of the combination of 90 nm front-side dielectric onrelatively thin piezoelectric substrates.

The dash-dot-dot horizontal line 850 marks the upper edge of band N77 at4200 MHz. The dash-dot vertical line 855 marks an LN-equivalentthickness of 518 nm, which is the thickness that provides a resonancefrequency of 3300 MHz for an XBAR with pitch equal to 10 times thepiezoelectric plate thickness. Reducing the pitch to 6 times thepiezoelectric plate thickness increases anti-resonance frequency byabout 3.2% compared to an XBAR with pitch equal to 10 times thepiezoelectric plate thickness. Conversely, an XBAR with pitch equal to 6times the piezoelectric plate thickness will need a 3.2% thickerdiaphragm to have the same anti-resonance frequency as an XB AR withpitch equal to 10 times the piezoelectric plate thickness. Assuming IDTpitch is not less than 6 times the piezoelectric plate thickness, themaximum LN-equivalent thickness for series resonators of a band N77 is518 nm x 1.032 or about 535 nm.

In a band N77 filter using a ladder filter circuit, the diaphragms ofall of the series resonators will have LN-equivalent thicknesses lessthan 535 nm. While there is no absolute minimum diaphragms thickness, atleast one series resonator in a typical band N77 filter will have anLN-equivalent diaphragm thickness greater than 465 nm to provide ananti-resonance frequency less than or equal to 4500 MHz (300 MHz greaterthan the upper band edge).

The LN-equivalent thickness of the diaphragm of the series resonatorsneed not be the same. One or more additional dielectric layer may beformed over a subset of the series resonators. The anti-resonancefrequencies of series resonators without the additional dielectric layerwill be further above the upper band edge (for example to increaseattenuation in a stop band above the upper band edge).

FIG. 9 is a schematic circuit diagram of an exemplary band N77 band-passfilter 900 using four series resonators Se1, Se2, Se3, Se4 and fourshunt resonators Sh1, Sh2, Sh3, Sh4. The filter 900 has a conventionalladder filter architecture. The four series resonators Se1, Se2, Se3,Se4 are connected in series between a first port P1 and a second portP2. The filter 900 is bidirectional and either port may serve as theinput or output of the filter. Shunt resonator Sh1 is connected fromport P1 to ground. The other three shunt resonators Sh2, Sh3, Sh4, areconnected from nodes between the series resonators to ground. All theshunt resonators and series resonators are composed of multiple XBARsub-resonators connected in parallel. The number of sub-resonators isindicated below the reference designator (e.g. “×4”) of each resonator.Dividing an XBAR into multiple sub-resonators has a primary benefit ofreducing the peak stress that would occur if each XBAR had a singlelarge diaphragm. The multiple sub-resonators of each resonatortypically, but not necessarily, have the same aperture and approximatelythe same length.

The pitch and mark of the sub-resonators comprising any of the series orshunt resonators are not necessarily the same. As shown in the detailedschematic diagram, series resonator Se3 is composed of foursub-resonators Se3A, Se3B, Se3C, Se3D connected in parallel. Each of thesub-resonators Se3A, Se3D has a unique mark and pitch, which is to saythe mark and pitch of any sub-resonator is different from the mark andpitch of each other sub-resonator. Co-pending application Ser. No.17/039,239, Transversely-Excited Film Bulk Acoustic Resonator WithMulti-Pitch Interdigital Transducer, describes the use of smallvariations (e.g. ±1%) in pitch within the IDT of an XBAR to reduce theamplitude of spurious modes. The small variations in pitch shift thefrequencies of spurious modes such that the spurious modes do not addconstructively over the area of the XBAR. These small pitch variationshave a negligible effect on the resonance and anti-resonancefrequencies. The small changes in pitch between the sub-resonators Se3A,Se3B, Se3C, Se3D have a similar effect. The spurious modes of thesub-resonators do not add, resulting in lower overall spurious modeamplitude. This technique is used in three of the four series resonatorsand three of the four shunt resonators of the filter 900.

The filter 900 uses a “split-ladder” architecture in which the seriesresonators Se1-Se4 are fabricated on a first chip 910 and the shuntresonators Sh1-Sh4 are fabricated on a second chip 920. The first chip910 and the second chip 920 are mounted on a circuit board 930 whichprovides interconnections between the first and second chips 910, 920.The first chip 910 and the second chip 920 include rotated y-cutpiezoelectric plates with different thicknesses.

The first chip 910 includes a frequency setting dielectric layer 915over the diaphragms of series resonators Se1-Se3, but not over thediaphragm of series resonator Se4. In other implementations of a BandN77 filter, both of the first chip 910 and the second chip 920 mayinclude a frequency setting dielectric layer over the diaphragms ofnone, some, or all of the resonators on the chip.

FIG. 10 is a table 1000 of parameters for the resonators of the filter900 of FIG. 9. The piezoelectric plate of the first chip 910, whichcontains series resonators Se1-Se4, is 127.5-degree YX cut lithiumniobate 455 nm thick. The IDT electrodes on the first chip 910 aresubstantially aluminum with a thickness of 670 nm. The first chip 910has a 20 nm passivation dielectric layer and an additional 40 nmfrequency setting dielectric layer over resonators Se1, Se2, and Se3.

The piezoelectric plate of the second chip 920, which contains shuntresonators Sh1-Sh4, is 127.5-degree YX cut lithium niobate 580 nm thick.The IDT electrodes on the second chip 920 are substantially aluminumwith a thickness of 680 nm. The second chip 920 has a 20 nm passivationdielectric layer over all resonators.

The table 1000 includes the pitch, pitch/piezoelectric plate thicknessratio p/tp, mark, and mark/pitch ratio m/p for all resonators. Averagevalues of pitch and mark are provided for resonators (such as resonatorSe3 as shown in FIG. 9) where the pitch and/or mark is varied betweensub-resonators. The p/tp ratio of all resonators falls within thepreferred range of 6 to 12.5 as previously described. The m/p ratio ofall resonators is with the preferred range of 0.2 to 0.3, as defined inco-pending application Ser. No. 17/030,066, TRANSVERSELY-EXCITED FILMBULK ACOUSTIC RESONATOR WITH REDUCED SPURIOUS MODES.

The filter 900 of FIG. 9 and FIG. 10 is exemplary. A band N77 bandpassfilter may have more or fewer then four series resonators, more or fewerthen four shunt resonators, and more or fewer than eight totalresonators. A band N77 filter may incorporate additional components suchas inductors or capacitors. Innumerable other filter designs usingresonator parameters other than those given in FIG. 10 may be possiblewith the limitations that the LN-equivalent diaphragm thickness forshunt resonators is greater than or equal to 556 nm, and theLN-equivalent diaphragm thickness for series resonators is less than orequal to 535 nm.

FIG. 11 is a graph of the performance of an exemplary split ladder bandN77 bandpass filter, which may be, or be similar to, the filter 900 ofFIG. 9 and FIG. 10. Specifically, the curve 1110 is a plot of themagnitude of the input/output transfer function S21 of the exemplaryfilter over a frequency range from 100 MHZ to 10 GHz. The dot-dash lines1120 and 1130 mark the lower and upper edge of band N77, respectively.

Description of Methods

FIG. 12 is a simplified flow chart showing a process 1200 for making awafer with a plurality of chips containing XBARs. The process 1200starts at 1205 with a device substrate and a plate of piezoelectricmaterial attached to a sacrificial substrate. The process 1200 ends at1295 with a plurality of completed XBAR chips. The flow chart of FIG. 12includes only major process steps. Various conventional process steps(e.g. surface preparation, cleaning, inspection, baking, annealing,monitoring, testing, etc.) may be performed before, between, after, andduring the steps shown in FIG. 12. With the exception of step 1280, allof the actions are performed concurrently on all chips on the wafer.

The flow chart of FIG. 12 captures three variations of the process 1200for making an XBAR which differ in when and how cavities are formed inthe device substrate. The cavities may be formed at steps 1210A, 1210B,or 1210C. Only one of these steps is performed in each of the threevariations of the process 1200.

The piezoelectric plate may be, for example, rotated YX-cut lithiumniobate. The piezoelectric plate may be some other material and/or someother cut. The substrate may preferably be silicon. The substrate may besome other material that allows formation of deep cavities by etching orother processing.

In one variation of the process 1200, one or more cavities are formed inthe device substrate at 1210A before the piezoelectric plate is bondedto the substrate at 1215. A separate cavity may be formed for eachresonator on the chip. The one or more cavities may be formed usingconventional photolithographic and etching techniques. Typically, thecavities formed at 1210A will not penetrate through the devicesubstrate.

At 1215, the piezoelectric plate is bonded to the device substrate. Thepiezoelectric plate and the substrate may be bonded by a wafer bondingprocess. Typically, the mating surfaces of the substrate and thepiezoelectric plate are highly polished. One or more layers ofintermediate materials, such as an oxide or metal, may be formed ordeposited on the mating surface of one or both of the piezoelectricplate and the substrate. One or both mating surfaces may be activatedusing, for example, a plasma process. The mating surfaces may then bepressed together with considerable force to establish molecular bondsbetween the piezoelectric plate and the substrate or intermediatematerial layers.

The sacrificial substrate may be removed at 1220, exposing a frontsurface of the piezoelectric plate. The actions at 1220 may includefurther processes, such as polishing and/or annealing, to prepare theexposed surface for subsequent process steps.

A conductor pattern, including IDTs of each XBAR, is formed at 1230 bydepositing and patterning one or more conductor layers on the frontsurface of the piezoelectric plate. The conductor layer may be, forexample, aluminum or an aluminum alloy with a thickness of 50 nm to 150nm. Optionally, one or more layers of other materials may be disposedbelow (i.e. between the conductor layer and the piezoelectric plate)and/or on top of the conductor layer. For example, a thin film oftitanium, chrome, or other metal may be used to improve the adhesionbetween the conductor layer and the piezoelectric plate. A conductionenhancement layer of gold, aluminum, copper or other higher conductivitymetal may be formed over portions of the conductor pattern (for examplethe IDT busbars and interconnections between the IDTs).

The conductor pattern may be formed at 1230 by depositing the conductorlayer and, optionally, one or more other metal layers in sequence overthe surface of the piezoelectric plate. The excess metal may then beremoved by etching through patterned photoresist. The conductor layercan be etched, for example, by plasma etching, reactive ion etching, wetchemical etching, and other etching techniques.

Alternatively, the conductor pattern may be formed at 1230 using alift-off process. Photoresist may be deposited over the piezoelectricplate. and patterned to define the conductor pattern. The conductorlayer and, optionally, one or more other layers may be deposited insequence over the surface of the piezoelectric plate. The photoresistmay then be removed, which removes the excess material, leaving theconductor pattern.

One or more optional frequency setting dielectric layers may be formedat 1240 by depositing and patterning one or more dielectric layers onthe front surface of the piezoelectric plate. The dielectric layer(s)may be formed between, and optionally over, the IDT fingers of some, butnot all XBARs. The frequency setting dielectric layer(s) are typicallySiO₂, but may be Si₃N₄, Al₂O₃, or some other dielectric material. Thethickness of each frequency setting dielectric layer is determined bythe desire frequency shift. In the example of FIG. 8 and FIG. 10, afrequency setting dielectric layer 40 nm thick is formed over the IDTsof XBARs SE1, SE2, and SE3.

A passivation/tuning dielectric layer is formed at 1250 by depositing adielectric material over all of the front surface of the piezoelectricplate except pads used for electric connections to circuitry external tothe chip. The passivation/tuning layer may be SiO₂, Si₃N₄, Al₂O₃, someother dielectric material, or a combination of two or more materials.The thickness of passivation/tuning layer is determined by the minimumamount of dielectric material required to deal the surface of the chipplus the amount of sacrificial material possibly needed for frequencytuning at 1270. In the example of FIG. 9 and FIG. 10, apassivation/tuning layer 20 nm thick (after tuning) is assumed over theIDTs of all XBARs.

In a second variation of the process 1200, one or more cavities areformed in the back side of the substrate at 1210B. A separate cavity maybe formed for each resonator on the chip. The one or more cavities maybe formed using an anisotropic or orientation-dependent dry or wet etchto open holes through the back side of the substrate to thepiezoelectric plate.

In a third variation of the process 1200, one or more cavities in theform of recesses in the substrate may be formed at 1210C by etching thesubstrate using an etchant introduced through openings in thepiezoelectric plate. A separate cavity may be formed for each resonatorin a filter device. The one or more cavities formed at 1210C will notpenetrate through the substrate.

In all variations of the process 1200, some or all of XBARs on each chipmay be measured or tested at 1260. For example, the admittance of someor all XBARs may be measured at RF frequencies to determine theresonance and/or the anti-resonance frequencies. The measuredfrequencies may be compared to the intended frequencies and a map offrequency errors over the surface of the wafer may be prepared.

At 1270, the frequency of some or all XBARs may be tuned by selectivelyremoving material from the surface of the passivation/tuning layer inaccordance with the frequency error map developed at 1260. The selectivematerial removal may be done, for example, using a scanning ion mill orother tool.

The chips may then be completed at 1280. Actions that may occur at 1280include forming bonding pads or solder bumps or other means for makingconnection between the chips and external circuitry, additional testing,and excising individual chips from the wafer containing multiple chips.After the chips are completed, the process 1200 ends at 1295.

FIG. 13 is a flow chart of a method 1300 for fabricating a split-ladderfilter device, which may be the split ladder filter device 900 of FIG.9. The method 1300 starts at 1310 and concludes at 1390 with a completedfilter device.

At 1320, a first chip is fabricated using the process of FIG. 12 with afirst piezoelectric plate thickness. The first chip contains one, some,or all of the series resonators of the filter device. The first chip maybe a portion of a first large multi-chip wafer such that multiple copiesof the first chip are produced during each repetition of the step 1320.In this case, individual chips may be excised from the wafer and testedas part of the action at 1320.

At 1330, a second chip is fabricated using the process of FIG. 12 with asecond piezoelectric plate thickness. The second chip contains one,some, or all of the shunt resonators of the filter device. The secondchip may be a portion of a second large multi-chip wafer such thatmultiple copies of the second chip are produced during each repetitionof the step 1330. In this case, individual chips may be excised from thewafer and tested as part of the action at 1330.

At 1340, a circuit card is fabricated. The circuit card may be, forexample, a printed wiring board or an LTCC card or some other form ofcircuit card. The circuit card may include one or more conductors formaking at least one electrical connection between a series resonator onthe first chip and a shunt resonator on the second chip. The circuit maybe a portion of large substrate such that multiple copies of the circuitcard are produced during each repetition of the step 1340. In this case,individual circuit cards may be excised from the substrate and tested aspart of the action at 1340. Alternatively, individual circuit cards maybe excised from the substrate after chips have been attached to thecircuit cards at 1350, or after the devices are packaged at 1360.

At 1350, individual first and second chips are assembled to a circuitcard (which may or may not be a portion of a larger substrate) usingknown processes. For example, the first and second chips may be“flip-chip” mounted to the circuit card using solder or gold bumps orballs to make electrical, mechanical, and thermal connections betweenthe chips and the circuit card. The first and second chips may beassembled to the circuit card in some other manner.

The filter device is completed at 1360. Completing the filter device at1360 includes packaging and testing. Completing the filter device at1360 may include excising individual circuit card/chip assemblies from alarger substrate before or after packaging.

Closing Comments

Throughout this description, the embodiments and examples shown shouldbe considered as exemplars, rather than limitations on the apparatus andprocedures disclosed or claimed. Although many of the examples presentedherein involve specific combinations of method acts or system elements,it should be understood that those acts and those elements may becombined in other ways to accomplish the same objectives. With regard toflowcharts, additional and fewer steps may be taken, and the steps asshown may be combined or further refined to achieve the methodsdescribed herein. Acts, elements and features discussed only inconnection with one embodiment are not intended to be excluded from asimilar role in other embodiments.

As used herein, “plurality” means two or more. As used herein, a “set”of items may include one or more of such items. As used herein, whetherin the written description or the claims, the terms “comprising”,“including”, “carrying”, “having”, “containing”, “involving”, and thelike are to be understood to be open-ended, i.e., to mean including butnot limited to. Only the transitional phrases “consisting of” and“consisting essentially of”, respectively, are closed or semi-closedtransitional phrases with respect to claims. Use of ordinal terms suchas “first”, “second”, “third”, etc., in the claims to modify a claimelement does not by itself connote any priority, precedence, or order ofone claim element over another or the temporal order in which acts of amethod are performed, but are used merely as labels to distinguish oneclaim element having a certain name from another element having a samename (but for use of the ordinal term) to distinguish the claimelements. As used herein, “and/or” means that the listed items arealternatives, but the alternatives also include any combination of thelisted items.

It is claimed:
 1. A band N77 bandpass filter comprising: a firstplurality of transversely-excited film bulk acoustic resonators (XBARs)on a first chip comprising a first rotated YX-cut lithium niobatepiezoelectric plate having a thickness less than or equal to 535 nm; asecond plurality of XBARs on a second chip comprising a second rotatedYX-cut lithium niobate piezoelectric plate having a thickness greaterthan or equal to 556 nm; and a circuit card coupled to the first chipand the second chip, the circuit card comprising conductors for makingelectrical connections between the first chip and the second chip. 2.The filter of claim 1, wherein the first chip, the second chip, and thecircuit card collectively form a ladder filter circuit, the firstplurality of XBARs are series resonators in the ladder filter circuit,and the second plurality of XBARs are shunt resonators in the ladderfilter circuit.
 3. The filter of claim 1, wherein the first and secondpiezoelectric plates have Euler angles [0°, β, 0°], where 30°≤β≤38°. 4.A band N77 bandpass filter comprising: a first chip comprising a firstsubstrate, a first rotated YX-cut lithium niobate plate (1^(st) LNplate), and a first plurality of acoustic resonators, each of the firstplurality of acoustic resonators comprising: an interdigital transducer(IDT) formed on the 1st LN plate, interleaved fingers of the IDTdisposed on a respective diaphragm, the diaphragm comprising arespective portion of the 1^(st) LN plate spanning a respective cavityin the first substrate and a dielectric layer formed on the 1^(st) LNplate between the interleaved fingers of the IDT, wherein anLN-equivalent thickness of the diaphragm is less than 535 nm; a secondchip comprising a second substrate, a second rotated YX-cut lithiumniobate plate (2^(nd) LN plate), and a second plurality of acousticresonators, each of the second plurality of acoustic resonatorscomprising: an interdigital transducer (IDT) formed on the 2^(nd) LNplate, interleaved fingers of the IDT disposed on a respectivediaphragm, the diaphragm comprising a respective portion of the 2^(nd)LN plate spanning a respective cavity in the first substrate and adielectric layer formed on the 2^(nd) LN plate between the interleavedfingers of the IDT, wherein an LN-equivalent thickness of the diaphragmis greater than 556 nm; and a circuit card coupled to the first chip andthe second chip, the circuit card comprising conductors for makingelectrical connections between the first chip and the second chip. 5.The filter of claim 4, wherein the first chip, the second chip, and thecircuit card collectively form a ladder filter circuit, the firstplurality of acoustic resonators are series resonators in the ladderfilter circuit, and the second plurality of acoustic resonators areshunt resonators in the ladder filter circuit.
 6. The filter of claim 4,wherein the 1^(st) LN plate and the 2^(nd) LN plate have Euler angles[0°, β, 0°], where 30°≤β≤38°.
 7. The filter of claim 4, wherein for allIDTs of the first plurality of acoustic resonators, a ratio of the pitchof the interleaved fingers to the thickness of the 1^(st) LN plate isgreater than or equal to 6 and less than or equal to 12.5, and for allIDTs of the second plurality of acoustic resonators, a ratio of thepitch of the interleaved fingers to the thickness of the 2^(nd) LN plateis greater than or equal to 6 and less than or equal to 12.5.
 8. Thefilter of claim 4, wherein for all IDTs of the first plurality ofacoustic resonators and the second plurality of acoustic resonators, aratio of the mark of the interleaved fingers to the pitch of theinterleaved fingers is greater than or equal to 0.2 and less than orequal to 0.3.
 9. The filter of claim 4, wherein the LN-equivalentthickness of the diaphragm of at least one of the first plurality ofacoustic resonators is greater than or equal to 465 nm.
 10. The filterof claim 4, wherein the LN-equivalent thickness of the diaphragm of atleast one of the second plurality of acoustic resonators is less than orequal to 600 nm.
 11. The filter of claim 4, wherein the LN-equivalentthickness of each diaphragm of the first plurality of acousticresonators is given byteq≈tp1+ks(td) where teq is the LN equivalent diaphragm thickness, tp1is the thickness of the 1^(st) LN plate, td is the thickness of therespective dielectric layer, and ks is a proportionality constant forseries resonators.
 12. The filter of claim 11, wherein the dielectriclayer is silicon dioxide and ks=0.45.
 13. The filter of claim 4, whereinthe LN-equivalent thickness of each diaphragm of the second plurality ofacoustic resonators is given byteq≈tp2+kp(td) where teq is the LN equivalent diaphragm thickness, tp2is the thickness of the 2^(nd) LN plate, td is the thickness of therespective dielectric layer, and kp is a proportionality constant forshunt resonators.
 14. The filter of claim 13, wherein the dielectriclayer is silicon dioxide and kp=0.59.